In recent years, it has been increasingly desired for silver halide photographic materials with improved photographic properties, such as sensitivity, graininess, gradation, sharpness, good keeping, and suitability to rapid processing especially of development. In particular, the demands for improving good keeping while minimizing fog and for further increasing sensitivity are strong.
Reduction sensitization has conventionally been studied for increasing sensitivity. Reduction sensitizers which have been proved useful for reduction sensitization of silver halide emulsions include stannous chloride (U.S. Pat. No. 2,487,850), polyamines or cyclic amine compounds (U.S. Pat. Nos. 2,518,698, 2,521,925 and 3,930,867), thiourea dioxide (aminoiminomethanesulfinic acid) type compounds (British patent 789,823 and U.S. Pat. No. 2,983,609 and 2,983,610), borane compounds (U.S. Pat. Nos. 3,779,777, 3,782,959, and 4,150,093), and ascorbic acid (EP 369491A). A comparative study of the silver nuclei formed by various reduction sensitization methods is described in Collier, Photographic Science and Engineering, Vol. 23, p. 113 (1979), in which the author uses dimethylamine borane, stannous chloride, and hydrazine as reduction sensitizers and adopts a high pH ripening method and a low pAg ripening method.
Strong reducing agents, such as hydrogen, SnCl.sub.2, amine boranes, and sodium borohydride, typically possess much more reducing power than would be required to create silver centers (R-centers) on a silver halide grain, and this feature often makes them difficult to control. The required stiochiometry of reducing agents is difficult to control and often gives over reduction which is readout as fog. The usual manifestation of this lack of control is "over-reduction" or creation of too large cluster, providing fog on the photographic element. Reduction of other emulsion components, including possible interactions with gelatin functionalities, adds further complications to the use of these agents. Even if successfully formed, reduction centers appear to often be unstable, and provide poor keeping.
So, reduction sensitization generally tends to cause noticeable fog when combined with gold sensitization, and a reduction sensitized emulsion has particularly poor keeping.
Accordingly, it has been keenly demanded to develop a method of reduction sensitization which provides a silver halide emulsion of low fog and satisfactory preservability.